The invention relates to nickel plated aluminum or aluminum alloy conductors. More specifically, it relates to nickel plating processes for aluminum or aluminum alloy conductors, and devices for implementing these processes. The invention also applies to electrical wires and cables with an aluminum or aluminum alloy core comprising at least one nickel plated conductor.
The word xe2x80x9caluminumxe2x80x9d covers aluminum and its alloys in the broad sense of the term. It will thus be used throughout the remainder of the text. The word xe2x80x9cconductorxe2x80x9d in this case refers to an electrically conducting body with an elongated shape and a length that is large compared to its transverse dimensions, such as a wire, strip, bar or tube.
Aluminum conductors are widely used for the transmission of electrical and/or thermal energy. These conductors are usually in the form of bars, flats, wires or cables when used as electrical conductors, and in the form of strips, bars or tubes when used as thermal conductors.
In particular, electrical wires and cables with an aluminum core, possibly including a coating made of an insulating material, are usually obtained from continuously cast and rolled xe2x80x9cmachinexe2x80x9d wire that is then drawn to the required diameter. Individual wires or strands can then be assembled together to form the conducting core of a cable.
Aluminum conductors may be used in the untreated state, in other words without any particular surface treatment of the conductor apart from possibly brushing the parts of the conductor on which electrical contact will be made, for most applications such as the transmission and distribution of electrical energy. However for some applications, it is preferable to coat the aluminum conductor with a nickel coat in order to improve the electrical contact properties.
In known continuous nickel plating processes, the conductor advances through at least one electrolytic nickel plating tank. A nickel electrode is installed in this tank and acts as an anode, and consequently it is connected to the positive terminal of an electrical power supply. The conductor to be treated acts as the pure cathode and consequently is electrically connected to the negative terminal of this power supply.
In French patent application FR 2,526,052 (corresponding to U.S. Pat. No. 4,492,615), the applicant proposed a process and a device for continuous electrolytic nickel plating of an aluminum conductor at advance speeds of 300 m/minute. According to this process, the electrolytic current is transmitted to the conductor through connection called a liquid current connection, in other words without any mechanical contact, which avoids the disadvantages of mechanical current connections, and particularly electrical arcs. More precisely, the conductor to be coated is moved inside a first tank in which there is a negatively polarized electrode, and then in a second tank in which there is a positively polarized electrode; an electrical current passes along the conductor as it advances in the bath. The first tank contains an aqueous ionic solution that transmits the electrical current from the electrode to the said conductor. The second tank contains the nickel plating bath.
However, nickel plating of conductors is an additional operation for which the cost should be minimized and the productivity maximized. For wire or cable conductors, satisfactory costs and productivity can be achieved by nickel plating individual wires while moving at high speed. However some markets, for example the aeronautical market, would like to use nickel plated aluminum wires with a diameter of between 0.1 and 0.5 mm, and cables made of such wires.
The method according to French patent application FR 2,526,052 cannot be used satisfactorily to nickel plate wires with a diameter of less than 1 mm with good productivity. The applicant has observed that the quality of the nickel coating is no longer sufficient when the advance speed is more than 20 m/minute. Furthermore, since the entire nickel plating current passes through the conductor to be treated, the risks of the conductor breaking during the treatment increase when the wire diameter drops to less than 1 mm for a given nickel coat thickness, so that the system is unusable if it is desirable to keep the advance speed high (and consequently also keep the nickel plating current high). Moreover, this solution imposes a current in the first tank equal to the nickel plating current in the second tank. The very high surface current density thus achieved causes severe attack on the conductor in the first tank and consequently surface irregularities on the conductor that make it more fragile. Finally, it has been found that the life of the baths is relatively limited in use, particularly due to the high current passing in the first bath and causing deposition of large quantities of precipitates.
In French patent application FR 2,646,174 (corresponding to U.S. Pat. No. 5,015,340), the applicant proposed to solve some of these disadvantages by using baths with an identical composition, one for the first step called the activation step and the other for the next nickel plating step, which is a means of keeping the conductor immersed during its passage from one tank to another. Admittedly, this solution makes it possible to reach advance speeds of the order of 130 m/minute, but it cannot limit the activation current intensity to the values strictly necessary since it is imposed by the intensity of the nickel plating current. This solution does not solve problems related to the liquid current connection.
In French patent application FR 2,609,292 (corresponding to U.S. Pat. No. 4,741,811), the applicant also proposed to modulate the current density along the conductor by reducing the current density in the part on the upstream side of the nickel plating bath and/or on the downstream side of the xe2x80x9cactivationxe2x80x9d bath, and by adjusting the acidity of the nickel plating bath to a pH value of between 1 and 5. In practice, this modulation is achieved by the use of series of electrodes and screens inserted between the electrodes and the conductor. This solution is a means of nickel plating wires with diameters between 0.51 and 0.15 mm at advance speeds of between 25 and 50 m/minute. However, this solution requires a complex device that requires a precise adjustment of the dimensions and the position of components, which in any case can change in time.
In French patent application FR 2,650,696, a process for continuous coating of an aluminum based conductor was proposed comprising a preliminary chemical treatment of the conductor surface to create bond points on the surface in the form of microscopic metallic germs, and deposition of a metallic layer on the conductor by electroplating. The process and the device described in this document present the disadvantages of operating at a low advance speed (immersion times are of the order of 20 to 24 seconds).
Therefore, the applicant searched for means of obtaining nickel plated aluminum conductors with a diameter of less than 1 mm that avoids the disadvantages of prior art while maintaining acceptable cost effectiveness and productivity with the lowest possible investment costs.
The purpose of the invention is a continuous (or xe2x80x9cdynamicxe2x80x9d) nickel plating process for an aluminum conductor.
More precisely, the continuous nickel plating process for an aluminum conductor according to the invention comprises a pre-treatment step P that improves the adherence of the nickel coat, and an electrolytic nickel plating step N, and is characterized in that the said pre-treatment P can also improve the contact properties of the said conductor sufficiently to enable a mechanical electrical contact, and in that the nickel plating current is transmitted to the said conductor through a mechanical electrical contact on the part of the conductor output from the pre-treatment step.
The electrolytic nickel plating step N forms a uniform nickel coat over the said conductor, by electroplating.
The applicant found that, unexpectedly, due to the pre-treatment operation, it was possible to use mechanical contacts on very small diameter conductors and transmit the entire nickel plating current into the conductor through these contacts (also called xe2x80x9ccurrent connectionsxe2x80x9d). The applicant also observed that, surprisingly, this solution could be used for advance speeds significantly greater than 20 m/minute, as illustrated in the example presented later.
Another purpose of the invention is a continuous (or xe2x80x9cdynamicxe2x80x9d) nickel plating device for an aluminum conductor.
Although the invention applies mainly to aluminum conductors intended for electrical applications, it is equally applicable to aluminum conductors intended for non-electrical uses such as thermal uses (that make use of the high thermal conductivity of aluminum, like a heat exchanger), or perhaps specifically mechanical uses.
The invention may also be applied to nickel plating of aluminum products such as wires, strips or tubes made of aluminum to be brazed. In particular, the purpose of the invention is to use the process or device according to the invention for nickel plating of an aluminum product so that it can be brazed. The nickel coat, typically of the order of 1 xcexcm thick, can be used to form a satisfactory brazed joint without the need for a special brazing flux.
Said aluminum products may be composite products comprising a base part and at least one clad alloy layer (also called brazing alloy). Said composite products are typically used in heat exchangers, especially in the automobile industries. The clad alloy is typically an aluminum-silicon alloy, which typically comprises between about 5 and 13 wt. % silicon (such as AA4343 and 4045 alloys). The nickel coat is deposited on the clad alloy.
A wetting agent may be added to the nickel coat or to the clad alloy layer, or both, in order to improve the wettability of the clad alloy during the brazing process. Said wetting agent is typically an element selected from the group consisting of lead, bismuth, lithium, antimony, tin, silver, thallium and any mixture thereof. When the wetting agent is added to the clad alloy layer the latter typically comprises between 0.01 and 1 wt. % of wetting agent. The wetting agent may be added to the nickel coat by electrolytically depositing both the nickel and the wetting agent. For that purpose the wetting agent may be introduced in the nickel-plating bath, typically as a compound of the wetting agent, such as acetates, citrates, sulfamates, fluoborates, lactates, oxides or mixtures thereof. For example, the following compounds may be used lead acetate, lead citrate, lead sulfamate, lead fluoborate, bismuth lactate or bismuth oxide. The amount of wetting agent compound in the plating bath is typically between 0.1 and 10 g/l.
The nickel coat allows flux-less brazing of magnesium containing composite products in controlled atmosphere brazing ovens (CAB ovens).
Another purpose of the invention is a process for the manufacture of an assembled product including the use of an aluminum product that has been nickel-plated according to the invention. The said manufacturing process may also comprise an operation for brazing the said nickel plated aluminum product. The assembled product may be a heat exchanger when the aluminum conductor is used as a thermal conductor.